Following in addition to the trend towards healthy products, natural products, and additive-free products, problems on the safety of foods, such as bovine spongiform encephalopathy (BSE), expectations for a yeast extract that is a natural seasoning have been rapidly increased. As a yeast extract, a nucleic acid-based extract in which a rich taste or an aftertaste is strengthened, and a peptide-based extract imparting a body or thickness of the taste to foods, which are recently attracting attentions, are actively developed.
On the other hand, as a method of adding a yeast extract in which umami having first taste is strengthened and which is represented by glutamic acid, a method of adding inexpensive purified sodium glutamate (MSG) from the outside is well known and examples for developing the yeast extract itself are not so many.
For example, as shown in Patent Document 1, there is a method of producing a yeast extract in which a glutamic acid content is enhanced by enlarging an amount of free glutamic acid accumulated in a yeast through mutation breeding, or as shown in Patent Document 2, there is also an attempt to enhance a glutamic acid content by keeping an extract extraction rate low and further by improving the production method such as treating with an enzyme.
However, the glutamic acid content in thus produced yeast extract is 14.5% or less converted into the content of a sodium salt thereof. Since the value is lower than that of a wheat gluten hydrolysate that is typically used as a glutamic acid-containing seasoning, it is not at a thoroughly satisfying level as for tastes.
Further, there are such problems as the lowering of the cell yield per sugar by a mutation breeding in Patent Document 1, and the limitation of the production method due to a low amount of glutamic acid accumulated in a yeast in Patent Document 2.
For developing a yeast extract having a high glutamic acid content by various production methods corresponding to a production design with productivity capable of meeting the industrial application, it is necessary to spectacularly enlarge the amount of glutamic acid accumulated in a yeast without lowering the productivity of the cell.
While sodium glutamate produced by using bacteria is accumulated outside of a cell, in the production of a yeast extract, glutamic acid is necessary to be accumulated within a yeast cell. However, it is conjectured that in the cell, a metabolism control system such as a feedback inhibition acts, so that it is not easy to accumulate glutamic acid in a high concentration within the cell.
There is such a report that by an analysis of the pool of free amino acids in a yeast cell, it is found that glutamic acid is present in large quantity in a cytoplasm and glutamine is present in large quantity in a vacuole (Eur. J. Biochem. Vol. 108, p. 439 (1980)). Thus, if a yeast accumulating in a high concentration, not only glutamic acid, but also glutamine that is differentially-localized can be developed, by an enzymatic conversion of glutamine, which is a usual method, the production of a yeast extract in which the glutamic acid content is extremely enhanced, is possible.
In Patent Document 3, by a gene recombination of Saccharomyces cerevisiae which is a monoploid laboratory yeast, a recombinant strain accumulating free glutamic acid and glutamine in a total amount of 5.4% is built and from this yeast, a yeast extract having a glutamic acid content of 16.2% is produced. However, even by using a gene recombination, yet the total amount of glutamic acid and glutamine is low and it is not more than that the glutamic acid content of the obtained yeast extract is a little over 20% converted into the content of a sodium salt thereof. In addition, the similar gene recombination by industrial yeasts with high ploidy and with a lowered sporulation ability or by Candida utilis yeast having no sporulation ability is difficult. Further, it is not a desirable method at present to apply the produced gene recombinant to foods, because there are barriers such as the control of a law and a resistance of consumers.
Thus, there is such a necessity that for producing a yeast extract containing glutamic acid in a high concentration, the gene recombination is not used, and a yeast accumulating free glutamic acid and glutamine in a high concentration in a cell by a natural mutation is bred.
Further, a synergistic effect of glutamic acid and 5′-guanylic acid or 5′-inosinic acid for the taste is well known and for producing a yeast extract having strong umami, it is preferred to use a strain containing besides the above amino acids, ribonucleic acid (RNA), which is a raw material for nucleotide, also in a high concentration.    Patent Document 1: Japanese Patent Application Publication No. JP-A-9-294581    Patent Document 2: Japanese Patent Application Publication No. JP-A-2006-129835    Patent Document 3: Japanese Patent Application Publication No. JP-A-2002-171961